92 MOHL ON THE STRUCTURE OF THE VEGETABLE CELL. 
Since the results of this investigation are treated of in the fol- 
lowing pages in more particular relation to Hartig’s memoir, a 
short abstract of the principal points ef his theory will not be 
considered superfluous. According to him, the cell-wall struc- 
ture consists of three superposed layers. The innermost, the 
ptychode, is the first developed; the ptychodes of young cells 
are consequently in immediate contact with each other, and 
cohere either at isolated, round spots, or in bands running in a 
spiral direction. At a later period, there separates, at the points 
where no cohesion has taken place, a mass which by solidifying 
becomes that layer which I have called secondary cellular mem- 
brane, and to which Hartig has given the name of Asthate ; lastly 
a common cement of both cells, the Eusthate, forms between the 
asthates of two neighbouring cells. This is the layer which has 
hitherto been regarded as the primary membrane ; in many cases 
it does not extend over the whole periphery of the cell, but is 
limited to points in the vicinity of an intercellular passage. These 
three layers differ in their behaviour towards iodine and sul- 
phuric acid. If, for instance, a cell be moistened with tincture 
of iodine, and then some dilute sulphuric acid allowed to act on 
it, the ptychode and the eusthate will be coloured yellow, while 
the asthate will become blue and swell considerably. This dif- 
ference of colour not only allows the ptychode to be distin- 
guished from the asthate, but shows also that the former clothes 
the dotted ducts. 
1. The Primordial Utricle. 
My observations were first directed to the detection of the 
ptychode. The following is the result :— 
If we examine a yearling shoot of a tree or the stem of an 
annual plant which, before the completion of its longitudinal 
growth, has been placed in spirit and kept there some time, we 
find in all the cells and vessels whose secondary layers have not 
attained their complete development, an inner membrane which 
is remarkably distinct from the other membranes of the cell. 
This membrane forms a completely closed, thin-walled, cell-like 
vesicle, which, in the fresh plant, is closely applied to the inner 
wall of the cell, and therefore escapes observation, but by keep- 
ing the specimen for some time in spirit becomes contracted 
and more or less detached from the cell-wall. In specimens 
